In motor-driven injection molding machines in which resin is plasticized by rotating a screw by a screw-rotating servomotor and at the same time is metered while the screw is advanced by an injection servomotor to inject the molten resin, it is known to apply a back pressure to the screw by the injection motor against a molten resin pressure acting on the screw in a screw receding direction, and control this back pressure to a target value to obtain a desired plasticized state of the resin when a metering process is in progress.
For example, during the metering, the output torque of the injection servomotor, which rotates in a screw advancing direction, is limited to a torque limit value corresponding to a target back pressure, such that when the molten resin pressure acting on the screw in the screw receding direction becomes higher than the target back pressure, the screw is allowed to move in the receding direction while rotating the injection motor reversely, thereby to control the back pressure to the target value by always balancing the molten resin pressure acting on the screw and the back pressure also acting thereon (see, for example, Japanese Patent Disclosure No. 60-262616). Another back pressure control is also known wherein the injection servomotor is rotated in the screw receding direction by a predetermined rotational angle every time the driving current supplied to the injection servomotor, which represents the output torque thereof, is increased by a predetermined value with an increase in the molten resin pressure. Further, there is still another back pressure control wherein the injection servomotor is rotated at a rotation speed corresponding to a predetermined screw receding speed, to control the output torque thereof to fall within a range corresponding to the target back pressure. Moreover, in another back pressure control method, the screw is receded at a low speed by the injection servomotor and at the same time the screw-rotating servomotor is operated such that the output torque thereof is within a predetermined range.
According to these prior art methods, however, the screw receding speed is occasionally discontinuously varied during the metering, making it unable to control the plasticized state of the resin to a desired one.